Magnetic field sensors can be used to sense an angle position of a shaft or other object. For example, a permanent magnet can be mounted on the shaft, and a magnetic field sensor can be arranged proximate to the magnet in order to sense a magnetic field generated by the magnet as it rotates with the shaft. When the magnetic field sensor is mounted next to the shaft at a particular distance from the axis of rotation of the shaft, the sensor is often referred to as an “off-axis” magnetic angular position sensor. Off-axis magnetic angular position sensors are often implemented when the front side of the shaft is inaccessible (e.g. due to a specific use or assembly of the shaft), and therefore sensor elements cannot be mounted on the axis of rotation. Conversely, an “on-axis” magnetic angular position sensor is mounted at or proximate to one end of the shaft facing its front side and, generally, in-line with or symmetrically to the axis of rotation. In some embodiments, on-axis magnetic field angle sensors can be designed to measure a magnetic field gradient. For this purpose, the magnetic field may be measured at two different points, which are arranged at opposing sides of and symmetrically to the axis of rotation. The gradient may then be determined in sufficient approximation for many applications by combining the two measured magnetic field values, e.g. by subtraction.
In many applications it is a general design goal for magnetic angular position sensors to be inexpensive while also being robust with respect to external magnetic fields and other disturbances and insensitive to assembly tolerances. One particular application for magnetic field angle sensors is in brushless DC (BLDC) motors for the detection of the (angular) shaft position during rotation. BLDC motors present a challenging environment for magnetic field sensors as they typically include strong rotating magnets and copper windings carrying large currents, both of which produce time-varying magnetic fields which interfere with the magnetic field used to measure angular position and thus result in a measurement error. These disturbing magnetic fields are strongly inhomogeneous, which makes it difficult to eliminate their effect on the angular position measurement. In view of these problems there is a general need for improvement in magnetic angular position sensors.